This blog discusses current weather, weather prediction, climate issues, and other topics

Wednesday, November 20, 2013

A Hard Freeze and Modified Arctic Air Hits the Northwest

The air felt very different today. Colder, drier and without a cloud in the sky. And it was different, as we switched the air mass over us from maritime polar to modified continental polar. The result will be the first hard freeze over the region, with 20s being common west of the Cascade crest and teens in colder, valley locations. Even colder in valley's east of the Cascade crest.

I am teaching atmospheric sciences 101 right now and the funny thing was that today my lecture was on air masses--large, relatively uniform blobs of air, thousands of km in dimension.

One air mass is continental polar: cold, dry air that forms over the arctic regions with a relatively uniform snow coverage...like Siberia and northern Canada. This air mass generally doesn't make it to western Washington because of the dual blocking effects of the Cascades and Rockies. This is the kind of air that often pushes southward into Montana, the Dakotas and the eastern two-thirds of the U.S. producing the frigid winter conditions they are famous for.

Another air mass is the one we normally "enjoy": maritime, polar air that starts as continental polar over Siberia, but then is warmed and moistened at low levels by the relatively warm waters of the Pacific Ocean. This air mass is mild, wet, and modestly unstable, with lots of shower activity. This is the air mass with our number on it, since our weather generally comes from the west.

But yesterday, something interesting happened. A large ridge over the eastern Pacific helped build a cold high pressure system over the Yukon and northern B.C., and then pushed this cold air southward across the Northwest U.S. Our winds switched to northerly, temperatures and dew points dropped, and the clouds melted away. The leading edge of the cold air is often called the arctic front by local meteorologists. We generally don't get the super-primo continental polar air because the Cascades and Okanogan terrain
keep the coldest air in B.C..

Let me illustrate all of this. Here is the forecast map of sea level pressure, surface winds, and lower atmosphere (about 3000 ft up) at 4 PM today (Wednesday). The purple and blues are cold air. You see the big pressure gradient at the leading edge of the cold air? Classic with an arctic front.

Last night was the first after the arctic front passage, and temperatures were noticeably cooler, with lots of freezing temperatures. But tonight (Wed/Thurs) will be much colder.

Here are last night's minimum temperatures over western WA. Forties near the water. Then low to mid-30s a bit away from H2O. But farther away from the water (like over east-side communities, SW Washington, etc.) there were lots of mid-20s. Move up into mountain valleys and teens were all over the place.

Like colder temperatures? No problemo. Look at NE Washington (see below). Lots of reports in the teens and single digits.

It will be much colder tonight (Wed/Thurs) since colder air is in place and the winds will be much lighter...thus less mixing of warmer air above. As of 9 PM Wednesday, regional temperatures were running 5-7F less than last night.

Bottom line: if you have any sensitive plants, particularly those in planters, you might want to cover them or bring them into a warmer place. Your tomatoes will be dead if any are still alive.

The probability of temperatures below freezing produced by the UW probcast forecasting system is shown next. Red and orange are pretty much certain (80% or higher probability). That is most of you. Only coastal areas have a chance of missing out on the chilly fun.

And here is probcast's predictions of minimum temperature. Around 25-26 F in Seattle. Lots of teens east of the Cascades.

And if you like to follow the cold temperatures around Seattle in real-time, including being able to see actual road surface temperatures as well, check out Seattle SnowWatch. Here are the latest temps (9:16 PM Wed). Blues indicate freezing and below. Lots of air temperatures (all at roughly 2-meters above the surface) in the 20s and subfreezing. Much cooler on the eastside. The road temps (in rectangular boxes) are all above freezing right now, but they will slowly cool during the night. Quite some contrasts: 42F near the water downtown to 27F in Woodinville. 15F. The contrast will be more extreme by daybreak.

By the way, looking for the perfect holiday at a modest price for the weather lover in your household. Look no longer! Students in my department, in concert with King-TV, have produced a Washington Weather Calender. Some of the proceeds goes to help support the UW student chapter of the American Meteorological Society, including helping them go to conferences. A good calendar with lots of local weather information, for a good cause. You can purchase them ($14.99 plus shipping) online here.

I bet you thought I was going to mention my book on Northwest weather.

4 comments:

Definitely cool and crisp out there today! Since it looks like we're in for a stretch of relatively sedate, if chilly, fall weather, how about writing one of those scenario posts? How about a blog on the mother of all snow events for the NW? How can we get historically cold (talking down to 0) air over our region and how would the Pacific cooperate to bring a lot of moisture and north winds? What would a January 1950 event (or an even more extreme one) look like? Thanks!